Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	$ 76,587	$ 20,275
Short-Term Municipal Bond Fund [Member] | Quoted Market Prices (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	20,520
Short-term investments		20,275
Short-Term Municipal Bond Fund [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Short-term investments		0
Short-Term Municipal Bond Fund [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Short-term investments		0
Short-Term Municipal Bond Fund [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	20,520
Short-term investments		20,275
Commercial Paper [Member] | Quoted Market Prices (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	13,971
Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Commercial Paper [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	13,971
Corporate Notes And Bonds [Member] | Quoted Market Prices (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	42,096
Marketable securities	76,325
Corporate Notes And Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Marketable securities	0
Corporate Notes And Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	0
Marketable securities	0
Corporate Notes And Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Short-term investments	42,096
Marketable securities	76,325
U.S. Government Agencies [Member] | Quoted Market Prices (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	13,154
U.S. Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	0
U.S. Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	0
U.S. Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	13,154
Quoted Market Prices (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	166,066
Short-term investments		20,275
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	0
Short-term investments		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	0
Short-term investments		0
Fair Value, Measurements, Recurring [Member]
Schedule of assets and liabilities measured at fair value on a recurring basis
Marketable securities	166,066
Short-term investments		$ 20,275